This invention pertains to that type of truck transport refrigeration unit typically used on medium-sized straight trucks for delivery of refrigerated loads to limited distances and for limited periods of time. The particular type of refrigerant unit with which this invention is concerned has a single refrigerant system circuit, but is provided with one refrigerant compressor driven by the truck engine, and a separate refrigerant compressor, usually called a standby compressor, located in the refrigerant unit itself as distinct from the truck engine compressor located in the truck engine compartment. The truck engine compressor is used when the truck is on the road makings its deliveries. The standby compressor, which is electrically driven from a source of electricity at the truck terminal, is used when the truck is at the terminal and is to have its load space refrigerated.
This general type of transport unit, that is one which uses two separate compressors, with a single circuit refrigeration system, is well known in the art. One problem that can be experienced with arrangements of this general type relates to one or the other compressors having an inadequate oil supply due to the oil being accumulated in the other compressor. Thus, if the compressor which has an inadequate oil supply is the particular compressor to be used, damage and/or destruction of that compressor with inadequate oil can result.
It is well understood in the refrigerant art that refrigerant oil is miscible with the refrigerant which, of course, is being pumped through the refrigerant system by the compressor. It is also understood refrigerant migration, carrying the oil with the refrigerant, occurs both due to gravity, and due to different temperatures prevailing in the system. Refrigerant will migrate from a higher temperature location to a lower temperature location. In a transport refrigeration system of the type with which this invention is concerned, different temperatures at different locations in the overall system naturally occur since the evaporator coil and associated elements are located in the load space of the truck, the condenser section including the condenser coil, the electrically driven standby compressor, and other associated elements is located exteriorly of the load space and typically on the front wall of the truck body, and the truck engine compressor is located in the truck engine compartment. Thus, it will be readily apparent that different temperatures can be experienced at these three different locations in accordance with various operational and temperature conditions.
In the prior art commercial arrangements of which we are aware, the two compressors used in the arrangements have had significantly different pumping capacities, as well as different oil capacities in the compressors. In one particular arrangement of which we are aware, the truck engine compressor has six times the pumping capacity of the electric standby compressor, even though the oil capacity of the truck engine compressor is less than the oil capacity of the electric standby compressor. The prior art arrangements have either used solenoid value to control the circuiting of the refrigerant flow in accordance with which compressor is operating, or have used check valves. It is our view that the use of check valves is superior to solenoid value since the rate of leakage through a solenoid value as compared with a check valve is in the order to 6 to 8 to 1. The prior art arrangement using check valves is considered inferior to our arrangement in that the force of gravity is in a direction aiding migration rather than opposing migration.
Thus, it is the aim of this invention to provide an overall refrigerant system arrangement of the two compressor type for a straight truck in which imbalances in oil for the two compressors due both to compressor operation and to oil migration is substantially prevented.